1. Field of the Invention
The present invention relates to a laser beam optical scanning device for writing data on an optical recording medium preferably for use in a variety of devices including a digital copying machine, a laser printer, an optical disk, and, in particular, to an optical scanning device that is capable of reducing variations in shading and increase light usage and improve reflectance during a scanning process.
2. Description of the Related Art
An optical scanning device for scanning a scanning surface aligned with the surface of an optical recording medium, such as a photosensitive material, is well known in relation to laser printers or the like. In an optical scanning device, a usual optical arrangement is such that a laser beam from a laser beam source is deflected by a light deflector, such as a rotating polygon mirror, and applied to the scanning surface as a light spot by a scanning lens. Thus, the incident angles of the laser beam on the reflective surface of the light deflector and on the scanning lens are caused to continuously vary during line scanning in the main scanning direction.
Reflectance of the deflecting reflective surface and reflectance and transmittance of the surface of the scanning lens vary in correspondence with the respective incident angles, so that the intensity of the light spot on the scanning surface generally fluctuates with an image height in the main scanning direction, resulting in an unevenness in image density in the line scanning or a deterioration in gradation. This phenomenon, which is referred to as "shading", is serious when the direction of polarization of the laser beam impinging upon the deflecting reflective surface is parallel with or perpendicular to the deflecting direction. Generally, the light intensity tends to be reduced or increased at either end in the main scanning direction with respect to the central image height in the main scanning direction.
To overcome this problem, a filter which has a transmittance distribution has conventionally been used in such an optical system to compensate for the variation in the intensity of light of the scanning line. However, there are limitations to the acceptable size and position of the filter. Further, use of such a filter results in an increase in cost of the optical scanning apparatus.
Japanese Patent Laid-Open No. 5-303049 proposed a construction in which a 1/4 wavelength plate is arranged in the optical path between the light source and the light deflector. Due to this construction, the polarization of the beam impinging upon the light deflector is converted to circularly polarized light, and reflectance of the reflective surface of the light deflector is kept substantially constant within the deflecting region, whereby a reduction in shading is realized.
However, the above-described construction uses a 1/4 wavelength plate, which is expensive, resulting in an increase in the production cost of the optical scanning device itself.